Yongxin Qiu

The investigation on carrier distribution in InGaN/GaN multiple quantum well layers

The carrier distribution and recombination dynamics of InGaN/GaN multiple quantum well (MQW) light-emitting diode structure are investigated. Two emission peaks were observed in the low temperature photoluminescence spectra of an InGaN/GaN MQW structure, but only one peak was observed in the electroluminescence (EL) spectra. Combined with the spatially resolved cathodoluminescence (CL) measurements, it is found that the electrically injected carrier distribution is governed by hole transport and diffusion in InGaN/GaN MQW structure due to the much lower mobility of hole. And the electron and hole recombination of EL occurs predominantly in the QWs that are located closer to the p-GaN layer.

Analysis of Modified Williamson-Hall Plots on GaN Layers

Williamson—Hall (W-H) analysis is often used to separate the lateral coherence length (LCL) broadening and dislocation broadening on the ω-scan with a Lorentzian distribution. However, besides the LCL broadening and dislocation broadening, curvature also can broaden the ω-scan peak. Usually, the ω-scan can be described by a Pseudo-Voigt (P-V) function more precisely than a Lorentzian function. Based on the P-V fit peak profile, we modify the W-H plots. Both LCL broadening and curvature broadening can be eliminated from (001) ω-scans plots simultaneously, and a reliable tilt can be obtained. This method is a good complementary for the existing method, but is more convenient. Although we focuse on GaN layers, the results are applicable to a wide range of other materials having mosaic structures.

Determination of the tilt and twist angles of curved GaN layers by high-resolution x-ray diffraction

The full-width at half-maximum (FWHM) of an x-ray rocking curve (XRC) has been used as a parameter to determine the tilt and twist angles of GaN layers. Nevertheless, when the thickness of GaN epilayer reaches several microns, the peak broadening due to curvature becomes non-negligible. In this paper, using the (0 0 l), l = 2, 4, 6, XRC to minimize the effects of wafer curvature was studied systematically. Also the method to determine the tilt angle of a curved GaN layer was proposed while the Williamson-Hall plot was unsuitable. It was found that the (0 0 6) XRC-FWHM had a significant advantage for high-quality GaN layers with the radius curvature of r less than 3.5 m. Furthermore, an extrapolating method of gaining a reliable tilt angle has also been proposed, with which the calculated error can be improved by 10% for r < 2 m crystals compared with the (0 0 6) XRC-FWHM. In skew geometry, we have demonstrated that the twist angles deriving from the (2 0 4) XRC-FWHM are in accord with those from the grazing incidence in-plane diffraction (IP-GID) method for significantly curved samples.

A practical route towards fabricating GaN nanowire arrays

GaN nanowire (NW) arrays have been fabricated by the electrodeless photoelectrochemical (PEC) etching method for the first time. Under appropriate conditions, the etching process is just a dislocation-hunted process, in which the etching solution “digs down” along the threading dislocations, resulting in the formation of GaN NWs by preferentially etching away the defective parts of GaN with dislocations and retaining the flawless parts. The NWs have a density of 1∼2 × 107 cm−2, diameters ranging from 150 nm to 500 nm, and corresponding lengths ranging from 10 μm to 20 μm. Transmission electron microscopy (TEM) indicates that these GaN NWs possess few dislocations. High resolution X-ray diffraction (HRXRD) and micro-Raman measurements show that these GaN NWs are stress-free. Room temperature cathodoluminescence (CL) measurements show a single near-band-edge emission at 367 nm with a full width at half maximum (FWHM) of 8 nm from the NWs, indicating a high optical quality. Additionally, negative piezoelectric current pluses are generated from the GaN NWs when the conductive atomic force microscope is scanned cross the arrays in contact mode. Such GaN NW arrays are promising building blocks for exploring nanodevices with excellent performance.

Growth Mechanism of Large-Size Rubrene Single Crystals Grown by a Solution Technique

Large-size and high-quality rubrene single crystals have been grown by a solution technique. The biggest crystal has 4 mm dimension. Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy both indicate that the crystals obtained are pure rubrene and did not incorporate the solvent at the detection level. The morphology of surface and transect of the rubrene single crystals was observed by optical microscope, atomic force microscope (AFM), and scanning electron microscope (SEM). Three-dimensional nucleation was happened on the surface of single crystals. The array of small and large straight steps was observed on the surface of the rubrene single crystals. Layer-like structures were also observed on the cross-section of single crystals. All results indicate that each large step is most probably formed from the piling of small steps and the growth mechanism of rubrene crystals grown from solution is probably a three-dimensional nucleation–layer-by-layer periods growth.

Structure, Stress State and Piezoelectric Property of GaN Nanopyramid Arrays

GaN nanopyramid (NP) arrays have been fabricated by a convenient electrodeless photoelectrochemical etching method. Transmission electron microscopy measurement indicates that these NPs are composed of crystalline GaN surrounding a dislocation. High-resolution X-ray diffraction and the micro-Raman spectrum reveal a highly compressive stress relaxation in the NPs compared with compressed GaN subfilm. Additionally, negative piezoelectric current pluses are generated from the GaN NPs when the conductive atomic force microscope scans cross the arrays in the contact mode. The result demonstrates that the GaN NP arrays are a promising candidate for nanogenerators.

Direct growth of GaN on sapphire with non-catalytic CVD graphene layers at high temperature

Today, heteroepitaxial GaN films on sapphire have focused on conventional two-step growth process using low temperature GaN buffer layer. Here, we show the direct growth of GaN films on sapphire by using a graphene layer at high temperature, which simplified the GaN growth process. The graphene is directly synthesized on non-catalytic sapphire substrate by chemical vapor deposition without problematic transfer processes, using C2H4 as a carbon source at the temperature of 1200 oC. The synthesized graphene has been characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM). We have compared the GaN grown on sapphire with and without graphene. The single crystal, smooth surface GaN films has been obtained on sapphire with graphene, and the nucleation of GaN films has been discussed. The GaN films illuminated high near-band-edge emission and good ultraviolet photosensor. It demonstrates that graphene is a potential, useful buffer layer for heteroepitaxy of high quality GaN films.